Analytical-industries GPR-28 Oxygen Analyzer Manuel d'utilisateur télécharger pdf (Page 25)

Advanced Instruments Inc.

The above times assume the introduction of a purge gas, the lower of the available zero or

sample (if known and constant) gas, with a ppm level O2 concentration less than 0.5 ppm is

introduced to the analyzer following span calibration to purge (accelerate the reaction of)

the O2 that has dissolved into the electrolyte inside the sensor. If zero gas is not available,

substitute the sample gas and expect slightly longer recovery time.

Span Gas Selection:

The O2 concentration of a span gas should approximate 70-90% of the full scale range

dictated by the span gas, e.g. 80 ppm O2 on the 0-100 ppm range. For optimum accuracy,

the full scale range dictated by the span gas should be at least one range higher than the

intended analysis range. Both of the aforementioned recommendations reduce the error

induced by the tolerance of the electronic components; also, span gases with higher O2

concentrations are more reliable and less expensive.

Conversely, if the recommended span gas is not available and air calibration is not an

option, a span gas of the same full scale range and near the anticipated analysis level

(approximately 10% of full scale) is acceptable with the understanding the accuracy will

suffer slightly.

Use of span gas near 10% of the same full scale range for measurements at the higher end

of the range has the effect of “expanding the error” by moving upscale as illustrated by

Graph A and Example 1 in the Accuracy section above and is not recommended. Of course

the user can always elect at his discretion to accept an accuracy error of +

2-3% of full scale

range if no other span gas is available.

Type of Sensor:

Pico-Ion MS Oxygen Sensors: Analyzers utilizing these sensors require span gas(es) and

cannot be calibrated with air (20.9% or 209,000 ppm O2) due to their high signal output.

Calibrate with a span gas containing less than 1000 ppm O2; a span gas containing 70-90

ppm O2 is recommended.

Galvanic Fuel Cell Sensors: Analyzers utilizing these sensors can be calibrated with ambient

air (20.9% or 209,000 ppm O2) and have been proven capable of precise analysis below

0.5 ppm (500 ppb).

Note: As described above, these oxygen sensors are capable of one point calibration.

Span Gas vs. Air

(Fuel Cell Sensors only):

Span gas calibration: Recommended for calibration of in-service analyzers, if the priority is

the fastest online recovery time.

Air calibration: Recommended for calibration an analyzer if a new oxygen sensor is being

installed or if the availability of span gas, the cost of span gas or the accuracy of a span gas

is an issue. An air calibrated analyzer can be used to reliably verify a “certified” span gas,

which has frequently been found to be inaccurate. For best results, select a recognized

name supplier.

Note: Galvanic Fuel Cell sensors can be calibrated (using air) and brought online (using

sample gas) without “purchased calibration gases” and without sacrificing accuracy -

provided the analyzer is given enough time to come down to the O2 concentration of the

sample gas.

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Analytical-industries GPR-28 Oxygen Analyzer Manuel d'utilisateur Page 25

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